1. Field of the Invention
This invention relates to xerographic imagers using a light emitter array. In particular, this invention is directed to architectures, characteristics and methods of using a fault-tolerant multibar arrangement for organic light emitting diode (OLED) printbars used in such xerographic light emitter arrays.
2. Technical Background
One of the fundamental design challenges for xerographic imaging is getting enough light to the photoreceptor at sufficient print speed while providing adequate service lifetime of the printbar. Rapid progress in OLEDs has produced devices which emit light levels greater than computer monitors (300 cd/m.sup.2) and fluorescent tubes (3000 cd/m.sup.2) in both white and in colors collectively spanning the visible spectrum.
Lifetime studies of OLEDs indicate that diode lifetime is determined by the total charge passed through the OLED. Thus the OLEDs operate for short times at high brightness or for long times at low brightness. The lower end of the OLED brightness range is most stable, generally sustaining lifetimes of greater than 10,000 hours. The higher end of the OLED brightness range is less stable. For example, OLED devices operating at 1500 cd/m.sup.2 currently have sustainable lifetimes of only about 500 hours.
In a one-dimensional page-width array of such OLEDs there is not enough brightness to print at a reasonable speed with reasonable reliability for commercial uses. Table 1 outlines the technical data for a xerographic printer using a single row OLED printbar having OLED emitters operating at 1500 cd/m.sup.2. The printbar is illuminating a photoreceptor requiring about 7.5 ergs/cm.sup.2. Thus, the print-speed of the single row device is about 0.29 pages/min. Moderate print-speeds are above five pages/min, and a more desirable print speed is about 30 pages per minute. The brightness deficit determined by this rough calculation is about 100.times., especially when considering that the print speed calculation for the single row page-width array of OLEDs leaves no room for dead time. In addition, inorganic diode based printbars typically have a duty cycle well under 50% in part to minimize blur in the process direction. Furthermore, the calculated print-speed is the speed before degradation, where the lifetime for the devices is the time to 50% output decay.
TABLE 1 __________________________________________________________________________ TECHNICAL DATA FOR A CONVENTIONAL SINGLE ROW OLED PRINTBAR Light Emitter Inputs Outputs __________________________________________________________________________ Average Wavelength 590 nm Surface Luminous Flux 0.4712 1 m/cm.sup.2 Avg. Luminous Efficacy 450 1 m/W Surface Radiance 0.0010 W/cm.sup.2 LED Brightness 1500 cd/m.sup.2 Surface Radiance 10472.0 ergs/sec.cm.sup.2 LED Current Density 25 mA/cm.sup.2 Photoreceptor Irradiance 103.778 ergs/sec.cm.sup.2 Display Voltage 20 Volts Pixel Size 0.0085 cm Number of Rows Pixel Current 1.79 uA Array Fill Factor 88% Array Emitting Area 0.26 cm.sup.2 Optical Inputs Array Width 0.08 mm Lens Transmittance 90% Array Emission 27.50 ergs/sec Lens Effective F# 4.765 Array Current 6.623643 mA Lens Efficiency 1.0% Array (Max) Power 0.13 Watts Photoreceptor Dose 7.5 erg/cm.sup.2 Power Efficiency 0.2094% Page Property Inputs.sup.2 Page Dose 5758.05 ergs Document exc. Time 0 sec Page Time 209.42 sec Fast scan resolution 300 in.sup.-1 Line Time 82.12 msec Slow scan resolution 300 in.sup.-1 Print Speed 0.287 pages/min Fast scan length 14 in Data Rate 0.051 MHz Slow scan length 8.5 in Fractional line Time 100% __________________________________________________________________________
The brightness deficit of currently available OLED devices is too large to compensate simply by running the diodes harder. For example, operating the OLEDs even briefly at 15000 cd/m.sup.2 would require such a high bias that the OLEDs would quickly become inoperative. Furthermore, doing so would only increase the print speed of the single row array to 3 pages/minute. In addition, the total lifetime print volume of the xerographic imager (&lt;9,000 pages) is insufficient.
Commonly assigned U.S. patent application Ser. No. 08/785,230 to Fork, filed concurrently herewith, entitled "Integrating Xerographic Light Emitter Array," the disclosure of which is incorporated herein by reference in its entirety, discloses one approach for using OLEDs operated at modest light levels to expose a photoreceptor drum or belt. This is accomplished by staging an array of emitters in the slow scan direction, and clocking the data through pixel driving shift-registers synchronized with the movement of photoreceptor past the array in the slow scan direction. Increased emitter lifetime and the ability to operate at lower light levels are achieved in proportion to the number of stages.
Commonly assigned U.S. patent application Ser. No. 08/785,233 to Fork et al., filed concurrently herewith, entitled "Integrating Xerographic Light Emitter Array with Grey Scale," the disclosure of which is incorporated herein by reference in its entirety, discloses another approach for using OLEDs operated at modest light levels to expose a photoreceptor drum or belt. This is accomplished by staging rows of emitters in the slow scan direction and moving the object image down the rows synchronously with the movement of the photoreceptor past the array in the slow scan direction in a manner similar to U.S. patent application Ser. No. 08/785,230. However, the entire printbar can be rewritten during each line time of the photoreceptor, which allows the exposure on any spot on the photoreceptor to be varied over a number of grey levels equal to the number of stages.